Electrical resistance alloy



United States Patent 3,171,737 ELECTRICAL RESISTANQE ALLOY I, Norman F. Spooner, Bloomfield Township, Oakland County, and Forbes S. Sibley, Birmingham, Niicim, assignors to HOSlfiiiS Manufacturing Company, Detroit, Mich, a corporation of Michigan No Drawing. Filed Apr.,28, 1961, Ser. No. 106,168 6 Claims. (Cl. 75124) This invention relates to electrical resistance alloys and more particularly to an electrical resistance alloy having a relatively low temperature coefiicient of resistivity and a modenate resistance.

The alloy is further characterized in that it can be drawn into extremely fine wire which is highly resistant to corrosion and the electrical properties of which are obtained without heat treatment after a conventional anneal. I

The electrical resistant alloy of this invention is extremely useful in the manufacture of electrical apparatus which requires resistors of extremely high accuracy such as those used in radio, television, radar, etc. While resistors of this type do not have to be particularly resistant to oxidation at high temperature since they never heat to a temperature substantially beyond 150 C., it is very important that they possess relatively high resistance to atmospheric corrosion and relatively high strength. The strength requirements of these resistors arise from the fact that such resistors are in the form of extremely fine wire, for example, 40 gage wire (.0'03" diameter) and also wire having a diameter as small as .001" or less; and these fine wires must possess suflicient strength to withstand coiling by conventional methods. In addition, such high accuracy resistors, if they are to retain their accuracy for a long period of time, must be highly resistant to corrosion under atmospheric conditions.

Experience has shown that 30-gauge wires of this alloy, having about one hundred and thirty thousand p.s.i. tensile strength as annealed will produce fine wires of about .001" dia. which will have strength sufficient to allow winding of resistors by conventional methods. Specifications for such resistors usually require a temperature coefiicient of resistivity (T.C.) of between $000020 ohm per ohm per C. in a temperature range of between 55 C. and 150 C. and a nominal resis tivity of about 800 ohms per circular mil foot (c.m.f.).

Heretofore, alloys designed for use as such high accuracy resistors having the necessary strength and corrosion resistance have been of the nickel-chromium type. For example, one such alloy has a nominal composition of 20% chromium, 74% nickel, 3% aluminum, and 3% copper. Another such alloy has a nominal composition of 20% chromium, 2% to 4% aluminum, up to 6% iron, and the balance nickel.

The alloy of this invention, while having substantially the same electrical properties as the two previously mentioned alloys over the temperature range of 55 C. to 150 C., possesses distinct advantages thereover. The previously mentioned alloys have a very high nickel content, a critical and expensive material. electrical properties referred to are developed in these alloys only after heat treatment. The alloy of this invention has a nominal composition of 5% aluminum, 4% molybdenum, 22% chromium, and the balance iron. Thus, in the alloy of the present invention, the nickel in the composition of the prior art alloys is replaced by iron; and furthermore, the present alloy possesses the desirable electrical properties without heat treatment. This latter feature is important in that by eliminating heat treatment, there is also eliminated slight variations in properties from one end of a spool of wire to the other because of slight non uniformities in temperature during heat treatment.

In addition, the

In addition, the alloy of this invention has a density approximately 10% less than the prior art alloys referred to which results in more feet of wire per pound. At the same time, by using molybdenum in the alloy in the range of 3% to 6%, the corrosion resistance and the strength of the alloy of this invention is equivalent to the two prior art alloys referred to. The alloy of this invention may have a nominal range of composition as follows: aluminum, 4.5% to 5.5%; chromium, to 30.0%; molybdenum, 3.0% to 6.0%; silicon, 1.0% maximum; carbon, .1% maximum; iron, balance. The preferred range of composition of the present alloy is as follows: aluminum, 4.6% to 5.3%; chromium, 20.0% to 25.0%; molybdenum, 3.5% to 50%; silicon, 0.5% maximum; carbon, .08% maxim'umfiron, balance. Deo'xidiz'ers such as cerium, calcium, and zirconium may be present in the alloy in small amounts and are included in the above compositions in the iron content. 7 r

The presence of carbon in the alloy is due primarily to its presence in the iron and to its possible use in deoxidation. The silicon in the alloy results primarily from its presence in the chromium and the iron and also in its pick up from the crucible when a crucible formed of a silicon bearing ceramic, such as, Zirconium silicate is employed. The presence of silicon may also result from the deox idizing practice employed.

The aluminum range in the composition of the alloy has been determined on the basis of the following eifects on the alloy:

(11) Its effect on the temperature coefiicient of resistivity;

([7) Its eliect on resistivity; and

(c) The eliects of the melting practice on the aluminum requirement. For example, the amount of silicon pick up will atfect the aluminum requirement.

The relationship between TC; and resistivity varies somewhat with melting practice; but in general, an increasing aluminum content increases the resistivity and lowers the TC. of the alloy. Alloys having a T.C. of i.000020 will have a resistivity in the range of 780 to 860 ohms pe'r c.m.f For a given melting practice, the relationship between resistivity, T .C. and aluminum con;- tent follows the regular pattern stated; namely, an in creasing aluminum content increases the resistivity and lowers the TC. w

The permissible range for the chromium content is rather broad becausethe properties or" the alloy do not vary as much with chromium concentration as they do with aluminum. Therange of the chromium content is determined by the T ,C. and resistiv ity requirements and by the limited workability of alloys with higher chromium contents. In general, the chromium content of the alloy affects its properties in a manner similar to aluminum, but to a much lesser degree.

The molybdenum content of the alloy is based upon proper balance between:

(a) The effect of molybdenum on strength; (12) The eifect of molybdenum 'on corrosion resistance;

and

(c) The eiiect of molybdenum on workability.

The tensile strength of the alloy increases with ah increase in moiybdenum content to a at about 3.5% to 4% molybdenum. An increase in the molybdenum content above 4% decreases the strength slightly. From the standpoint of strength alone, 3.5% to 4% molybdenum is optimum. With respect to corrosion resistance, this increases rapidly for the first 25% molybdenum and beyond that amount, increases at a decreasing rate as the molybdenum content is increased to about 5%. Beyond 5% .3 molybdenum, there is no substantial increase in the alloys resistance to corrosion.

The molybdenum content of the alloy adversely affects its workability. However, with as much as 4% molybdenum, the alloy can be drawn to fine wire by normal processes; and at 6% molybdenum, the alloy can still be drawn, but with some difiiculty. Above 6% molybdenum, the alloys cannot be drawn in accordance with conventional processes. Thus, the lower limit of the molybdenum in the composition is set at 3% molybdenum to provide adequate strength and corrosion resistance combined with a high degree of workability. The upper limit of molybdenum in the alloy is set at to assure adequate strength and adequate workability. For extremely fine wire having a diameter of .003 or less, the upper limit of molybdenum in the alloy is set at 4% to enable adequate workability.

The preferred method of preparing the alloy comprises first melting high purity iron, chromium and molybdenum together at a reduced pressure in a suitable crucible, preferably magnesium oxide, to prevent pick up of secondary elements. After the oxygen content of the melt has been reduced by a suitable deoxidizing agent or agents, such as carbon, calcium, cerium or zirconium, high purity aluminum is added to the melt and allowed to mix with the alloy. The melt is then cast into suitable molds and ingots are worked to a final form such as wire by the normal process; that is, rolling and drawing. As is conventional, the wire is subjected to a final standard anneal to obtain the best temper for winding into resistors. The annealing can be done in a continuous manner at a temperature of between 1500 to 1800 F. in a non oxidizing atmosphere to preserve its bright finish.

It has been found that differences in the melting practice, such as the crucible material, the size of the heat, the type of deoxidizer and the type of atmosphere, can affect the properties of the alloy. For example, an alloy melted in air in a zirconium silicate crucible as a 480 lb. heat with calcium deoxidation, and having an aluminum content of 4.5%, had substantially the same TC. and resistivity as a 46 lb. heat melted in vacuum in a'z'irconium silicate crucible with cerium deoxidati'on and having an aluminum content of 4.9%, the remainder of the composition being substantially the same for both alloys. As another example, one alloy containing 4.9% aluminum in a 46pound ingot melted in the vacuum in a zirconium silicate crucible had substantially the same T.C. and resistivity as another alloy containing 5.3% aluminum in a 95-pound ingot melted in vacuum in a magnesium, oxide crucible, the remainder of the composition being substantially the same for both alloys. The heat melted in zirconium silicate picked up silicon, which lowered the aluminum requirement. The ranges set forth above take into account the effects produced by different melting practices.

Below are set forth, in table form, a few examples of alloys of this invention and the properties of such alloys:

4 heat treatment a temperature coefiicient of resistivity of 1.000020 ohm per ohm per C. within the range of C. to 150 C. and a resistivity of about 800 ohms per circular mil foot at 25 C., said alloy consisting essentially of 4.5 to 5.5% aluminum, 15% to 30% chro= mium, 3.0% to 4.0% molybdenum and the balance essentially iron.

2. A high strength, corrosion resistant electrical resist ance element in the form of wire having a diameter of about .003" or less comprising an alloy producing without heat treatment a temperature coefiicient of resistivity of :000020 ohm per ohm per C. within the range of 55 C. to 150 C. and a resistivity of about 800 ohms per circular mil foot at 25 C., said alloy consisting essentially of 4.6% to 5.3% aluminum, 20% to 25% chromium, 3.5 to 4.0% molybdenum and the balance essentially iron.

3. A high strength corrosion resistant electrical resistance element in the form of wire having a diameter of about .003" or less comprising an alloy producing without heat treatment a temperature coelficient of resistivity of :.000020 ohm per ohm per C. Within the range of 55 C. to 150 C. and .a resistivity of about 800 ohms per circular mil foot at 25 C., said alloy consisting essentially of 4.5% to 5.5% aluminum, 15% to 30% chromium, 3.0% to 4.0% molybdenum, 1% maximum silicon, .l% maximum carbon, and the balance essentially iron.

4. A high strength corrosion resistant electrical resist ance element in the form of wire having a diameter of about .003" or less comprising an alloy producing without heat treatment a temperature coefiicient of resistivity of *-.000020 ohm per ohm per C. and a resistivity of about 800 ohms per circular mil foot at 25 C., said alloy consisting essentially of 4.6% to 5.3% aluminum, 20% to 25% chromium, 3.5% to 4.0% molybdenum, 1% maximum silicon, .l% maximum carbon, and the balance essentially iron.

5. A high strength corrosion resistant electrical resistance element in the form of wire having a diameter of about .003 or less comprising an alloy producing without heat treatment a temperature coefiicient of resistivity of :000020 ohm per ohm per C. within the range of -55 C. to 150 C. and a resistivity of about 800 ohms per circular mil foot at 25 C., said alloy consisting essentially of about 5% aluminum, 4% molybdenum, 20% to 25 chromium and the balance essentially iron.

6. A high strength corrosion resistant electrical resistance element in the form of wire having a diameter of about .003" or less comprising an alloy producing without heat treatment a temperature coefficient of resistivity of -.000020 ohm per ohm per C. within the range of -55 C. to 150 C. and a resistivity of about 800 ohms per circular mil foot at 25 C., said alloy consisting essentially of about 5% aluminum, 4% molybdenum, 20% to 25% chromium, 1% maximum silicon, .l% maximum carbon, and the balance essentially iron.

'1 M Afinealed D T Al Mo Cr Zr Si 0 (-55/25 0. 051 i; niii str ii tii (p.p.m.l 0. ohms] (p.s.i.)

GJl'Lf.

Various melting practices were employed in these examples. However, they were all annealed in the same References Clted m the file of thls patent manner at approximately the same temperature. TED STATES PATENTS This application is a continuation-in-part of our copend- 1,333,723 Rudgr Nov, 24 1931 ing application Serial No. 801,722, filed March 25, 1959.

wa lin h 1 1 FOREIGN PATENTS g strengt corrosion resistant e ectrica resist- 680 930 ance element in the form of wire having a diameter of 617194 3322 335 3525 7 about .003" or less comprising an alloy producing without 

1. A HIGH STRENGTH, CORROSION RESISTANT ELECTRICAL RESISTANCE ELEMENT IN THE FORM OF WIRE HAVING A DIAMETER OF ABOUT .003" OR LESS COMPRISING AN ALLOY PRODUCING WITHOUT HEAT TREATMENT A TEMPERATURE COEFFICIENT OF RESISTIVITY OF $.000020 OHM PER OHM PER *C. WITHIN THE RANGE OF -55*C. TO 150*C. AND A RESISTIVITY OF ABOUT 800 OHMS PER CIRCULAR MIL FOOT AT 25*C., SAID ALLOY CONSISTING ESSENTIALLY OF 4.5% TO 5.5% ALLUMINUM, 15% TO 30% CHROMIUM, 3.0% TO 4.0% MOLYBDENUM AND THE BALANCE ESSENTIALLY IRON. 